The mystical energy of all the world. In Aikido you extend your ki into your opponent. It is a gift. Uke should see it that way. Your ki is to be seen as a gift, as should ukes. Uke gives forward ki, you take his ki as a gift and you turn it into your ki, throwing uke with his and your ki, in this process you both should reach and have a higher understanding of all the universe. Ki is strength, ki is life. Ki is the power to heal everything.